Clinical and laboratory data have converged on the idea that drug addiction is the usurpation of synaptic plasticity that normally supports reward-related learning and memory. For example, repeated cocaine exposure induces multiple forms of synaptic plasticity in the neuronal circuitry for reward learning. Synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength. It includes presynaptic change on the quantity of neurotransmitter released and postsynaptic change on how effectively cells respond to those neurbtransmitters. Our view of post -synaptic plasticity is greatly advanced owing to pharmacological and electrophysiological tools with high selectivity and sensitivity. However, presynaptic plasticity, also closely involved in addiction, is less appreciated, in part due to the lack of ways to directly monitor synaptic vesicles releasing neurotransmitter. Taking advantage of its nanoscale size and superior photoproperties, I developed a quantum -dot-based approach that can directly report the behavior of single presynaptic vesicles in cultured hippocampal neurons for minutes or even hours. I discovered that vesicles with high release probability prefer a mode of fast and reversible reuse (kiss-and-run, K&R) and the potentiation of neurotransmission was achieved by the increase of K&R. To understand how the fusion and retrieval of synaptic vesicles and the presynaptic release of neurotransmitter are altered in drug addiction, I propose to: (1) investigate the modification of vesicle release associated with presynaptic plasticity, (2) identify the protein regulators of vesicle release that mediate presynaptic plasticity, and (3) illustrate how vesicular release of dopamine is regulated and how addictive drugs affect it. Given the importance of synaptic vesicles in presynaptic plasticity and the significance of presynaptic plasticity in drug addiction, understanding how synaptic vesicles are malfunctioned in drug addiction may prove beneficial in the treatment of one of society's most intractable health problems. PUBLIC HEALTH REVELANCE: This project aims to explore the molecular and cellular mechanisms that regulate neurotransmission in the presynaptic terminals and to understand how they are perturbed by addictive substances. It will advance our knowledge about the neurological foundation of drug addiction, which is well aligned with the mission of the National Institute of Drug Abuse.